Ample research has demonstrated that exposure to inadmissible evidence affects decision-making in criminal and civil cases. However, the difficulty of ignoring information in the context of legal interpretation has not been examined yet. Our study addresses the possible effects that exposure to preparatory work has on the interpretation of treaties. In the present article, we examine the ability of students enrolled in international law courses and of international law experts to ignore preparatory work when they are not allowed to use it. We found that exposure to preparatory work affected the students’ interpretation of treaties, while no such effect was found among the experts. These results reaffirm the practical relevance of the debate over the hierarchy between the rules of treaty interpretation. In particular, our study demonstrates that preparatory work can play a significant role in decision-making, depending on the legal rule that applies to the use of such materials. More generally, our study suggests that legal interpretation by students and experts is qualitatively different, and that international law experts might be better able than non-experts to discount irrelevant information in the process of treaty interpretation.